The manufacture of semiconductors typically requires precise control of fluids (e.g., gasses, liquids, gas-vapor mixes) through the process path to a process chamber. Regulation of mass flow is often achieved through the use of a thermal mass flow controller. These mass flow controllers generally operate by directing fluid flow through a capillary tube path that runs parallel to a bypass area of the main flow path. Two thermally sensitive resistors are wound around the capillary tube. As fluid travels through the capillary tube, heat is conducted away from the resistors, or imparted to the fluid, causing the resistance of each of the resistors to change and the temperature of the fluid to change. Based on the difference in resistance between the two resistors, a controller, executing a control algorithm can determine the flow through the mass flow controller. Many mass flow meters work on a similar principle.
Thermal mass flow controllers and meters, such as those described above, are subject to a phenomenon known as “thermal siphoning” wherein heated fluid in the capillary tube initiates convective flow with the cooler fluid in the main flow path. While this flow may be small, it can cause measurable changes in the zero of the mass flow controller. This problem is exacerbated if the attitude (i.e., angle from the horizontal of the main flow path) changes. Because of thermal siphoning, prior art mass flow controllers must typically be calibrated on a controller-by-controller basis for the orientation in which each mass flow controller will be installed.
Several schemes for attitude compensation have been developed to try to offset the effects of thermal siphoning. U.S. Pat. No. 5,763,774 describes a system for minimizing thermal siphoning by reducing the internal volume of the capillary tubes and bypass area. While this method reduces the effects of thermal siphoning, a small amount of thermal siphoning remains.
Another prior art system for addressing thermal siphoning is to increase the flow restriction in the bypass area and/or the capillary tube area, as described in U.S. Pat. No. 6,763,774. This method can adversely affect sensing in some applications because it increases the differential pressure required across the sensing element.
U.S. Pat. No. 6,044,701 describes another prior art method for addressing thermal siphoning. In this scheme, the capillary tube is orthogonal to the main axis of flow through the flow measurement/control device. This method requires a complicated flow path that is difficult to manufacture. Moreover, this method simply reorients the axis of the capillary tube ninety degrees and will experience the same difficulties as other flow measurement devices if installed in an orientation that is rotated ninety degrees around the main axis of flow relative to the intended installation orientation.
U.S. Pat. Nos. 4,100,801 and 4,956,975 describe a method for reversing the flow path. While this system acts to reduce the effects of thermal siphoning, mass flow measurement/control devices incorporating this system can be more expensive to manufacture. Moreover, reversing the flow path, or even making the flow path orthogonal to the main flow path through the thermal mass flow sensor, can increase the internal volume of the thermal mass flow sensor. The increase in internal volume can make the device unsuitable for use in systems with low vapor pressure.